1. Field of the Invention
The present invention relates in general to a presbyopia correction contact lens having a near vision correction region and a distance vision correction region, and more particularly to such contact lens having concentric correction areas of different correction powers.
2. Discussion of the Prior Art
There have been proposed multi-focus contact lenses for vision correction of a patient's eye having weakened accommodation faculties such as presbyopia. Such contact lens has a multiplicity of correction powers or a continuously changing correction power.
Such multi-focus contact lenses are divided into two major types. The first type is adapted such that a near vision correction region and a distance vision correction region are alternatively used as needed by the lens wearer, while the second type is adapted such that the near and distance vision correction regions are simultaneously used so that the desired distance from the wearer's eye is selected by an action of the wearer's brain to observe an object at that distance. Since the first type of lens requires sophisticated technology for processing the lens, and since the alternative use of the different correction regions for observation of the desired object is difficult, the second or simultaneous vision type adapted to observe the object through the two correction regions is currently prevailing.
Examples of the simultaneous vision type contact lens are disclosed in GB-A-2086605, U.S. Pat. No. 4,636,049, EP-A-201231 and U.S. Pat. No. 4,752,123. These publications propose various types of presbyopia correction contact lens, each of which has concentric near vision correction region and distance vision correction region that have different correction powers. However, these contact lenses are not necessarily satisfactory in their functions of corrections for both near and distance visions.
To find out a mechanism responsible for the above drawback encountered on such conventional presbyopia correction contact lenses, the present inventors made specimens of various types and experiments on patients' eyes. Extensive study and analysis based on the experiments revealed that most of the lens specimens were displaced or shifted on the eyes toward the corresponding ear of the lens wearer during use. It was also discovered that this tendency of displacement of the contact lens arises from the configuration of the cornea in general. Namely, the analysis indicated that the cornea of the human eyes has a larger curvature at a portion on the side of the ear than a portion on the side of the nose, so that the contact lens is more likely to be moved toward the portion of the cornea on the side of the corresponding ear.
On the other hand, it is known that the center of the pupil of the human eyes in general is offset about 0.2-0.6 mm from the center of the cornea toward the nose.
However, the known contact lenses are designed such that the concentric correction regions have a common center at the center of a circle defining the periphery of the lens (i.e., geometrical center of the lens), on the assumption that the geometric center of the lens is to be aligned with the pupil, while in fact the lens tends to be displaced toward the ear and the pupil is generally offset toward the nose. Consequently, the individual correction regions of the contact lens do not have a predetermined positional relationship with respect to the pupil of the eye, whereby the known contact lenses make it difficult for the lens wearers to obtain clear images through the near vision correction and distance vision correction regions.
A further research by the present inventors regarding the positional relationship between the correction regions of the contact lens and the pupils of the eyes revealed that near objects may be viewed even if the near vision correction region of the lens does not cover the entirety or the most of the area of the pupil, or distant objects may be viewed even if the distance correction region of the lens does not cover the entirety or the most of the area of the pupil.